Linear low drop-out voltage regulators (LDOs) are widely used in supplying electronic systems. Their design methods are highly mature. Stability issues are resolved, and fully integrated on-chip versions are helping in the build of complex circuits on a chip without external components (U.S. Pat. No. 6,960,907). One major drawback, however, for such regulators is their efficiency being low. This is the case when the difference between input voltage and regulated output voltage is large.
In order to overcome efficiency, switching regulators have been developed. Switching regulators are based on storing and releasing electrical energy with a controlled duty cycle. Since the switches are completely on or off, these types of regulators do not suffer from wasted energy from the regulating pass transistors of the LDOs which operate in linear mode.
However, energy storage elements used in switching regulators have a few drawbacks. They have undesired parasitics which causes reduction in efficiency. They are also expensive. Most of all, as of yet there is no feasible technique to integrate these components onto the same chip where the rest of the electronic system resides.
One possible solution to integrate the energy storing elements onto a into chip is to increase the switching frequency. High switching frequencies allow these elements to be small in size. One problem is that complex pulse width modulation (PWM) servo mechanisms that are usually based on a sawtooth signal, cause extreme phase shift along the feedback. As frequency increases, stabilizing such a complex system becomes cumbersome. It is possible to simplify the servo system by using bang-bang types of designs. This would deteriorate the performance of the regulator.
Another solution is to make a digital servo mechanism by which stability issues can be partly resolved using elegant digital algorithms. Since an analog to digital converter (ADC) is required to measure the output voltage, the speed of ADC will be the limiting factor of the switching frequency. However, such system gets even more complex and expensive.
The idea of combining LDOs and switching regulators is not new. A discussion was done in part of Isik and James's article “Power Reduction Techniques in Mixed Signal Integrated Circuits: Practical approaches” at Page 3. However, all the prior art known to the inventor suggests that the use of switching regulator in front of the linear regulators will have all disadvantages mentioned above. Some embodiments consist of a switching regulator and a following LDO (U.S. Pat. No. 7,084,612, US20050242792, US20090261790, U.S. Pat. No. 5,592,072), or with another LDO parallel to the switching regulator (U.S. Pat. No. 7,230,408, U.S. Pat. No. 6,984,969) or another LDO (U.S. Pat. No. 6,809,504). These prior art solutions were expected to be fully integrated, since the switching frequency cannot be as high enough, however, full integration is not trivial. For example, in U.S. Pat. No. 7,084,612, feedback from current flow information is used to generate a PWM signal, however, no integration operation which may be essential to stability, is mentioned. Instead, a bang-bang type operation that is not always desirable for various reasons, is suggested.
More importantly, when a linear regulator follows a switching regulator, it is not possible to combine the linear regulator's pass transistor and the high side switch transistor in one. The latter is very important to increase the efficiency further. One of the prior art described in U.S. Pat. No. 6,150,798, makes double use of a high side switch for both linear and switching supply circuits. The usage of a high side switch in this prior art is not continuous. Instead, the mode of operation is selected by external control signals based on input voltage or a sleep condition.
Some recent embodiments take advantage of the fact that many output voltages are needed at the same time. With an elegant design described in Le et. al., only one inductor may serve for producing many voltages without sacrificing efficiency.
The goals of these mentioned prior art were to increase the performance and the efficiency. Although some suggest simplification to a certain extend, they do not bring any significant cost reduction.